Injection mold dies for synthetic resins may comprise multiple cavities, each having an associated channel feeding resin to the cavity from a central manifold. When it is desired to use only selected cavities, it is necessary to block access channels to those cavities which are not to be used. One known method for blocking such channels is to mill a slot transversely of a die channel, drill and tap the die at the bottom of the slot on either side of the channel, and fit the slot with a rectilinear bar secured to the die by screw fasteners positioned in the drilled and tapped holes. To open the channel for subsequent use, the bar must be replaced with a similar member in which a channel opening has been cut. Such gates are relatively time-consuming and expensive to install, and occupy a relatively great amount of space in the mold.